I was under the impression that it was down to personal preference.
Some people won't use anything they didn't see drop in a CUK game.
Some will only accept kit from CUK players for fear of it being duped or hacked.
To me, if you trade for an item with an item you believe to be legit, who cares if the item you trade for is a dupe? So long as the item isn't obviously hacked, you're making a fair trade and not cheating the system in any way shape or form.
Your only concern should be if the item you traded for will go pop next time Blizzard have a clear out.
That's the way I see it.